In general, a liquid crystal display apparatus displays images by light from a backlight apparatus which has a plurality of fluorescent lamps such as cold cathode tube lamps. An inverter circuit is generally used in a lighting circuit. Light emission efficiency of the lamps is enhanced by lighting the lamps at a high frequency, thus making it possible to miniaturize electric components such as a transformer.
The liquid crystal display apparatus has been required to have higher efficiency as they become increasingly popular. In particular, the backlight apparatus consumes a major part of power for the liquid crystal display apparatus, and accordingly enhancement of the efficiency thereof is important. In this regard, it is proposed to reduce a switching loss of the inverter circuit by adopting a soft switching method such as zero voltage switching for a discharge lamp lighting circuit. It is also proposed to reduce a loss of a step-up transformer by using a direct current power supply of a high voltage specification to output to the inverter circuit.
As light sources of the liquid crystal display apparatus, not only the fluorescent lamps but also LEDs as semiconductors which emit light, organic ELs, and the like are being put into practical use. Hence, the efficiency enhancement is also required for the lighting circuit that supplies power to these light emitting devices.
However, efficiency of power conversion in the lighting circuit for these backlight apparatuses is lower than that of a lighting circuit for general illumination. This is because a direct current stabilizer circuit is separately required for suppressing flickering as compared with the lighting circuit for the general illumination.
Here, it is shown why the flickering is not regarded as a problem in the lighting circuit for the general illumination. For example, flickering in a fluorescent lamp inverter circuit is considered when a commercial power (50 Hz) is used as a power supply. In the fluorescent lamp inverter circuit, a rectifier circuit rectifies the commercial power, a smoothing circuit smoothes a direct current voltage from the rectifier and supplies direct current power to the inverter circuit, and thereby the inverter circuit supplies high frequency power to each of the lamps. The output voltage of the smoothing circuit contains a so-called ripple voltage. That is, it does not become a complete direct current.
Therefore, an output voltage of the inverter circuit also fluctuates to some extent by the ripple voltage. For example, it is assumed that the direct current voltage inputted to the inverter circuit contains a ripple voltage with 100 Hz. This 100 Hz is a frequency component of the commercial power, and it appears when the commercial power is converted with full-wave rectification. When the inverter circuit lights the lamp without removing this ripple voltage, fluctuations having the frequency component of 100 Hz occur in a light output. In the inverter circuit for the general illumination, a level of the ripple voltage is approximately 10% or less. Even if one directly seen this light, the one hardly sensed flickering therein. This is because such a frequency of the light fluctuation is as high as 100 Hz. Therefore, this flickering is not particularly regarded as a problem in the inverter circuit of the fluorescent lamp, which is used in the general illumination and the like.
However, in the liquid crystal display apparatus, a lighting method is different from that for the general illumination, and accordingly, the flickering becomes significant. This is because the general backlight apparatus of the liquid crystal display apparatus performs impulse lighting at a relatively low frequency.
Patent Literature 1 (Japanese Patent Laid-Open Publication No. H07-272889), a period of applying a high frequency voltage to the fluorescent lamp and a period of not applying the high frequency voltage thereto are periodically repeated, whereby dimming of the fluorescent lamp is performed. In this dimming, the light output is determined from a time ratio between such a lighting period and such a shut-off period. Hence, the light output is linearly changed in comparison with a method of continuously varying a lamp current. Moreover, blinks of the fluorescent lamp improves a blur of a moving picture on the liquid crystal display apparatus.
Patent Literature 2 (Japanese Patent Laid-Open Publication No. H11-202286) discloses a technique for obtaining a clear image by impulse light emission of a light source in the liquid crystal display apparatus. This technique allows the light source to emit light in matching with an update cycle of an image, and thereby improves responsiveness of the liquid crystal display apparatus, of which slowness is a disadvantage. Specifically, when updating a display image at 60 Hz, the light source just performs the impulse light emission at 60 Hz. As described above, the impulse light emission of the light source is useful for the liquid crystal display apparatus.
However, this impulse light emission requires a stable power supply. This is because, when the power supply of the light source contains the ripple voltage of the commercial power, fluctuations occur owing to interference between a frequency of the ripple voltage and a frequency of the impulse light emission. For example, it is assumed that the frequency of the commercial power is 50 Hz, and that a ripple voltage with 100 Hz is generated in the output of the smoothing circuit. If the frequency of the impulse light emission is assumed to be 120 Hz at this time, light fluctuations of 20 Hz occur. Specifically, when a difference between the frequency of the commercial power and the frequency of the impulse light emission becomes a low frequency, then such a difference appears as the flickering.
As countermeasures against this flickering, there is a method of setting the frequency of the impulse emission away from the ripple frequency. Specifically, setting is made so as to increase the difference between the frequency of the commercial power and the frequency of the impulse light emission. When the ripple frequency and the frequency of the impulse light emission is set at 100 Hz and 380 Hz, respectively, the difference between both of the frequencies becomes 280 Hz, and thereby the flickering becomes inconspicuous.
However, in order to efficiently obtain such a clear image as in Patent Literature 2, it is necessary to synchronize a cycle of the impulse light emission with the update cycle of the display image. Specifically, when a liquid crystal display updates images at 60 Hz, the frequency of the impulse light emission can be selected from among 60 Hz, 120 Hz and 180 Hz. In this case, a frequency as low as possible must be selected in order to obtain the clearest image with the brightest screen.
In this connection, a general liquid crystal display apparatus includes a regulated power supply circuit that removes the ripple voltage of the direct current power, which is caused by the commercial power. If there is no influence by the ripple, then the problem of the flickering caused by the ripple does not occur. However, regulating the power supply increases a circuit loss. Accordingly, efficiency of conversion from the power to the light in the liquid crystal display apparatus decreases.
Incidentally, as a method of reducing the influence of the ripple without using the regulated power supply circuit, it is conceived to add a function to remove the power supply ripple to the inverter circuit. For example, the lamp current is used in feedback control. If the feedback control is performed, the lamp current becomes substantially constant, and the fluctuations of the light output by the ripple voltage can be removed.
However, in this feedback control, it is necessary to detect the lamp current by an isolated secondary side circuit, and to transmit a detection signal thus obtained to a non-isolated primary side switching circuit. Hence, it is actually difficult to design the feedback control for objects including a transmission circuit concerned.
As another method, it is conceived to perform feedforward control. In this control, an inverter output is increased and reduced in response to the ripple voltage. This enables to configure a control circuit at the non-isolated primary side, and thus it becomes easy to design an isolation circuit. Further, since the ripple voltage is generated relatively stably, and it is suitable for the feedforward control. In this regards, various methods for this feedforward control for the discharge lamp lighting apparatus are proposed.
Patent Literature 3 (Japanese Patent Laid-Open Publication No. 2002-330591) proposes a technique for suppression of the change of the lamp current owing to the voltage fluctuations. This technique detects an input voltage of the inverter, and changes a drive frequency of a switch or a time ratio between switching on and switching off so as to perform the suppression as described above.
Moreover, Patent Literature 4 (Japanese Unexamined Patent Publication No. 2007-529872) proposes a technique for the feedforward control. This technique accumulates energy in an inductor and the like and performs the feedforward control so that energy accumulated in a converter giving energy to the light source can be constant.